As is well known, light enters the eye through the cornea. Besides serving as a protective covering for the front of the eye, the cornea also helps focus light on the retina located at the back of the eye. After passing through the cornea, light enters the pupil located In the middle of the iris.
Behind the iris sits the lens. The lens divides the eyeball of mammalian into two segments, each filled with fluid. The anterior segment extends from the cornea to the lens. The posterior segment extends from the back edges of the lens to the retina. The anterior segment itself is divided into two chambers. The anterior chamber extends from the cornea to the iris while the posterior chamber extends from the iris to the lens.
The anterior segment is filled with a fluid called the aqueous humor that nourishes its internal structures. The posterior segment contains a gel-like substance called the vitreous humor. These fluids help the eyeball maintain its shape.
The mammalian crystalline lens typically has a natural elasticity and, in its relaxed state, assumes a substantially bi-convex configuration wherein it has a generally circular cross-section of two convex refracting surfaces. Generally, the curvature of the posterior surface of the lens, i.e. the surface adjacent to the vitreous body, is somewhat greater than that of the anterior surface.
The lens is generally located on the optical axis of the eye, i.e. in a substantially straight line drawn from the centre of the cornea to the macula on the retina at the posterior portion of the globe. The lens is closely surrounded by a membranous capsule that serves as an intermediate structure in the support and actuation of the lens. The lens capsule has transparent elastic anterior and posterior walls or capsular membranes. The lens and its capsule are suspended on the optical axis behind the pupil by a circular assembly of cobweb-like and radially-directed collagenous fibers commonly referred to as the zonules, that are attached at their inner ends to the equatorial region of the lens capsule and at their outer ends to the ciliary body, a muscular ring of tissue located just within the outer supporting structure of the eye, known as the sclera.
By changing its shape, the lens focuses light onto the retina. Normally, the eye creates a clear image because the cornea and lens refract incoming light rays to focus them on the retina. The shape of the cornea is fixed, but the lens changes shape to focus on objects at various distances from the eye. The diopter value of a lens is defined as the reciprocal of the focal lens, i.e.:Diopter=1/focal length (m)
Focal length is the distance from the centre of the lens to the object being viewed. The focal length must decrease as magnification Increases. The diopter value expresses the refractive capacity of a lens which is associated with the radius of curvature of the optics. Generally, an increased diopter value indicates that the optic is thicker and also has a lesser radius of curvature, thus possessing greater light bending capability.
During a process known as accommodation, the shape of the lens is altered and, hence, its refractive properties thereby adjusted, to allow the eye to focus on objects at varying distances. A typical healthy eye has sufficient accommodation to enable focused vision on objects ranging in distance from infinity, which is generally defined as beyond 20 feet from the eye, to very near, i.e. typically closer than 10 inches.
Accommodation occurs when the ciliary muscle moves the lens from its relaxed or “unaccommodated” state to a contracted or “accommodated state”. The ciliary body or muscle is relaxed in the unaccommodated eye and, therefore, assumes its largest diameter. When the viewer is observing an image located at a distance, the sensory cells within the retina signal the ciliary body to relax.
Accommodation occurs when the ciliary muscle moves the lens from its relaxed or “unaccommodated” state to a contracted or “accommodated” state. When the viewer is observing an image located at a distance, the sensory cells within the retina signal the ciliary body to relax. Conversely, the ciliary muscle is contracted in an effort for the eye to be focused on a near object.
Various accommodation theories have been proposed over the years. Probably, the most widely held theory of accommodation is that proposed by Helmholtz. According the Helmholtz theory, when focusing at near, the contraction of the ciliary muscle decreases the equatorial circumlenticular space. This, in turn, reduces the tension on the zonules and allows the lens to round up, hence increasing optical power. When viewing a distant object, the relaxation of the ciliary muscle increases the equatorial circumlenticular space causing an increase in zonular tension. The increase in zonular tension, in turn, causes the surfaces of the lens to flatten and the optical power of the lens to decrease.
According to the Schachar theory, while tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing.
According to the Coleman theory, the lens, zonules and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape in the mechanically reproducible state of a steep radius of curvature In the centre of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross-section, of a catenary.
According to Young, there is a pressure increase in the vitreous chamber under accommodation and convergence, and this increase in pressure is directly related to the amount of accommodation extended. Young feels that it is difficult to explain this pressure increase using the Helmholtz theory. A possibly more compatible explanation will be that of Tschering, claiming that the ciliary muscle, in pulling on the choroid, presses the vitreous, the ciliary body and the posterior part of the zonules against the lens, which is thus forcibly altered in shape.
According to Coleman, neither of these two theories of accommodation fully explains how the eye responds during accommodation. Coleman's model of the accommodative mechanism explains accommodation as a function of both lens plasticity and vitreous support based on analysis of hydraulic forces in the eye, showing that active vitreous support is consistent with decreased zonular tension and that the two theories are not contradictory.
The present invention takes into consideration the well-observed phenomena that there seems to be an increase in vitreous pressure during accommodation, possibly in relation with contraction of the ciliary muscle. Also, there seems to be a decrease in equatorial circumlenticular space upon muscle contraction.
Regardless of the theory, the natural accommodative capability thus involves contraction and relaxation of the ciliary muscles by the brain to alter the shape of the lens to the appropriate refractive parameters for focusing the light rays entering the eye onto the retina in order to provide both near and distant vision.
The natural accommodative capability thus involves contraction and relaxation of the ciliary muscles by the brain to alter the shape of the lens to the appropriate refractive parameters for focusing the light rays entering the eye on the retina in order to divide both near vision and distant vision.
There are multiple reasons for loss of accommodation. Typically, starting at age mid-40, a typical human eye begins to gradually lose its near distance vision, a process also referred to as presbyopia. One of the reasons is that the crystalline lens may become too hard to change back and forth from a thin lens to a thick lens.
The common approach for addressing such problem of loss of accommodation is to wear reading glasses. The prior art has also shown various attempts to solve presbyopia by implanting intra-ocular lenses.
Another conventional use for intra-ocular lenses is the replacement of the natural lens by an intra-ocular lens during cataract surgery. Indeed, in response to various physiological conditions, the most notable being the occurrence of cataracts, the natural crystalline lens may have to be removed and replaced by an intra-ocular lens.
The prior art discloses various examples of so-called monofocal intra-ocular lenses. Such monofocal intra-ocular lenses are produced with a single vision correction power, that being a vision correction power typically for distance vision. However, such conventional monofocal intra-ocular lenses typically do not provide sufficient near vision correction for reading or other situations where near vision is required.
The prior art also discloses so-called multifocal intra-ocular lenses that are produced with a lens providing more than one optical power, for example, a distance vision correction power and a near vision correction power. Although such multifocal intra-ocular lenses have proven to be relatively effective in providing the desired range of vision correction power, they may not be totally acceptable to some patients due to the simultaneous vision characteristics of such lenses which may produce halo and/or glare and/or decreased contrast sensitivity phenomena.
In order to provide patients with a range of vision correction powers with reduced risks of producing halo or glare phenomena, so-called accommodative intra-ocular lenses have been suggested in the prior art. Such prior art accommodative intra-ocular lenses may provide, for example, for the axial movement of the monofocal optic in order to vary the focus of an image on the retina. Most prior art accommodative intra-ocular lenses, however, suffer from some drawbacks.
For example, some prior art accommodative intra-ocular lenses are often limited by the amount of movement required to produce adequate accommodation.
Another problem encountered with the use of some prior art accommodative intra-ocular lenses is often referred to as posterior capsule opacification resulting from cell growth from the capsular bag unto the optics of the accommodative intra-ocular lens resulting from the presence of then latter. Such posterior capsule opacification can interfere with the clarity of the optic to the detriment of the vision of the lens wearer.
U.S. Pat. No. 4,253,199 to Banko approaches the problem of providing a focusable intra-ocular lens by providing a replacement intra-ocular lens of deformable material sutured to the ciliary body. This intra-ocular lens functions in much the same manner as the natural crystalline lens, but may cause bleeding because it requires sutures.
In an attempt to circumvent some of the problems associated with prior art lenses, U.S. Patent application Publication No. U.S. 2007/0129801 A1 naming J. Stuart Cummings as inventor and published Jun. 7, 2007 discloses an accommodative intra-ocular lens having an optic formed of solid silicone and liquid silicone. The optic is substantially circular and has a posterior portion of solid silicone extending into an anterior annular portion of solid silicone forming an outer diameter portion extending from the posterior side to an anterior side of the lens. The optic also includes a solid central anterior portion extending from the anterior annular portion which includes a membrane substantially thinner than the solid posterior and solid annular portions. The membrane is capable of deformation. The optic also includes a liquid silicone retained therein by the aforesaid solid portions. The optic is designed so that the anterior portion can change in radius of curvature upon an increase in vitreous cavity pressure on the posterior solid portion.
Although circumventing some of the problems associated with the prior art, the accommodative intra-ocular lens disclosed in this publication nevertheless suffers from some drawbacks. One of the main drawbacks associated with the structure disclosed in the Cumming application resides in the fact that the configuration of the anterior and posterior portions of the lens is such that the solid portions of the lens increase the risks that the latter creates refractive aberrations such as halo, glare and reduced contrast sensitivity. Indeed, for example, the configuration of the anterior portion of the lens is such that portions Identified respectively by the reference numerals 12b and 12d will deform substantially differently upon an increase in vitreous cavity pressure in part due to the relatively abrupt change in thickness at their juncture. The portions identified by the reference numerals 12a and 12b are designed so as to be sufficiently solid to prevent deformation of the optic upon implantation into the eye. This rigidity will also potentially contribute to the creation of refractive aberrations during use.
Against this background, there exists a need for an improved accommodative intra-ocular lens.